Metal-graphite brushes used in a conventional motor are manufactured by carrying out mixing, pressure forming, and baking processes on graphite as a brush material, pitch or synthetic resin powder as a binder, metal powder such as copper powder or silver powder, and, as necessary, an additive such as molybdenum disulfide that acts as a solid lubricant. When the included amount of metal powder is large, there are also cases where a binder is not used.
Also, the metal brushes of a motor and the electrical contacts of a relay are manufactured by fixing a contact member of silver-palladium, gold-silver, or the like onto a conductive spring member of phosphor bronze or the like, or by punching out a clad member where a contact member and a spring member are bonded together.
The electrical contacts of connector terminals are formed by tin plating the entire surface of a conductive spring member or gold plating only the contacts so as to reduce the contact resistance and to suppress the formation of an oxide film.
Conventionally, the selection of a material for electrical contacts is determined with overall consideration to the magnitude of the current flowing through the contacts, the contact resistance between the contact points, and the environment resistance with respect to a sulfide gas atmosphere and the like, with it presently not being possible to select an appropriate material without conducting experiments.
Out of conventional electrical contact members, motor brushes and relay contacts that are placed in moving contact are often designed and used with the premise of becoming worn so that an oxide film can be removed from the surface of the contact points.
Deterioration and wear of the contacts occur in the following ways: adhesive abrasion that occurs due to the materials composing the contacts adhering and the adhering parts being pulled off; abrasive abrasion where a hard material such as an oxide is produced either on the surfaces of one or both of the sliding surfaces of the contacts, or in the gap between both contacts, and soft components are scraped due to a blade-like effect of this hard material; arc abrasion, such as metal transfer and vaporization, that accompanies melting of the metal powder composing the sliding surfaces due to an electrical arc that occurs at the contacts; and erosion of the graphite composing the brushes and carbides in the binder that accompanies an overheating of the sliding parts.
It should be noted that to solve the above problems, the present applicant has developed brushes including carbon fibers made up of carbon nanofibers or carbon nanotubes and already filed a patent application (Japanese Patent Application No. 2002-189706).
The graphite used in conventional electrical contacts has a layered crystal structure and is anisotropic in that the electrical conductivity in a direction between the layers is remarkably small compared to the electrical conductivity in the same direction as the layers. For this reason, the contact resistance for graphite-graphite and graphite-metal greatly varies according to the contact direction for the graphite. For example, as shown in FIG. 9, since a sufficient current flows only in the direction of the layers in the graphite 8, current flows only in the direction shown by the arrow in FIG. 9, resulting in a loss in efficiency. It should be noted that reference numeral 5 designates the brush, 6 the commutator, and 7 copper powder. In addition, since there are hollows and many protrusions, the contact area with the commutator 6 is small and the sliding characteristics are poor regardless of a construction including graphite, so that the abrasive abrasion, arc abrasion, and erosion described above are likely, which results in a shorter working life. For this reason, to extend the working life, it is necessary to increase the length of the brushes and/or to use a brush replacement mechanism for regularly replacing the brushes. This results in an increase in the size of the part of the motor to which the brushes are attached and in increased importance for the motor user cleaning or replacing the brushes.
Also, when metal is used as the electrical contacts, contact resistance increases due to oxidization of the metal surface. When a noble metal is used, there are the problems of an increase in cost and of the metal composing the contacts melting and being vaporized due to an electrical arc. In the worst case, welding also occurs at the contacts.
It should be noted that as described in Japanese Patent Application No. 2002-189706, when carbon nanofibers or carbon nanotubes are simply mixed into a material, it is not always easy to stably disperse the carbon fibers in a contact layer.
The present invention was conceived to solve the problems described above, and it is an object of the present invention to provide an electrical contact member where wear can be reduced.